I believe what you need to do is to convert the displayed/generated pixel values, which are greyscale values, into the corresponding Hounsfield Unit (HU). To do this you use the following equation:

HU = pixel_value*slope - intercept

The intercept is found in the dicom header at 0028,1052 and the slope at 0028,1053. Plug these into the equation and you obtain the HU value which is simply a rescaling of the linear attenuation coefficient.